The present invention relates generally to film thickness measurements, and more specifically to a film thickness measurement technique that measures surface conductance.
The industry of semiconductor manufacturing involves highly complex techniques for integrating circuits into semiconductor materials. Due to the large scale of circuit integration and the decreasing size of semiconductor devices, the semiconductor manufacturing process is prone to processing defects. Testing procedures are therefore critical to maintain quality control. Since the testing procedures are an integral and significant part of the manufacturing process, the semiconductor industry constantly seeks more accurate and efficient testing procedures.
A critical aspect of semiconductor fabrication involves the formation of the multiple conductive layers and liner layers. Each conductive layer includes the metal traces, also referred to as interconnects, which form the paths along which electronic signals travel within semiconductor devices. Dielectric material layers and liner layers separate conductive layers. The dielectric material layer, commonly silicon dioxide, provides electrical insulation between the conductive layers. Portions of each conductive layer are connected to portions of other conductive layers by electrical pathways called xe2x80x9cplugs.xe2x80x9d The liner layers are formed between each conductive layer and each dielectric material layer to prevent the conductive material from diffusing into the dielectric material layer. The liner layer inhibits a conductive layer from diffusing into an underlying dielectric and shorting circuiting with an adjacent 30 conductive layer. Of course, such short circuit formations are likely to be detrimental to semiconductor performance. In particular note, copper, a common conductive material used in semiconductor devices, diffuses very aggressively into silicon dioxide. The thickness and composition of the conductive and liner layers must be formed under extremely small margins of error. Thus, systems capable of testing the characteristics of these layers arc very important.
Some of the current methods for measuring conductive film stack characteristics include eddy current testing, microwave probe testing, four-point probe testing, X-ray fluorescence testing, and photo-induced surface acoustic wave testing. Unfortunately, each of these methods has associated disadvantages that limit their usefulness. Specifically, the eddy current probes have difficulty resolving small variations in thin film thickness and also have spot sizes that are relatively large in comparison to areas of interest on a semiconductor device. Eddy current probes have low resolution for thin films for at least a couple of reasons. First, conventional eddy current probes use large inductor coils (in the mm range) that are driven at relatively low frequencies (kHz range). Such probe systems create electromagnetic fields that penetrate too deep to resolve the thickness of thin films. Secondly, film thickness measurements by eddy current probe systems are based upon impedance values of the probe. Unfortunately, the probe""s impedance is not sufficiently sensitive to film thickness to provide high resolution of thin films.
Microwave probes are generally formed of a conductive core positioned within a cylindrical housing. The probe is driven at its resonant frequency such that a standing wave is created in the cylindrical housing and such the probe absorbs a maximum amount of energy from the signal generator. Measurements are made by placing the probe next to a specimen and measuring the change in the reflected signal and the resonant frequency shift. The reflected signal is the strength of a signal emitted from, or reflected out of, the probe due to the fact that the interaction between the specimen and the probe reduces the probe""s ability to absorb energy from the signal generator. The interaction specifically involves the conductance of the specimen changing the inductance and resistance of the microwave probe. The resonant frequency shift is the shift in the reflected signal""s frequency that occurs when the probe is placed next to the specimen. Even though microwave probes operate at high frequencies and therefore have high resolutions for particular thin films, the usefulness of these probes are limited because they are not sensitive to high conductance materials. This is particularly limiting considering the fact that copper has a high conductance value and is commonly used in thin film applications.
A disadvantage of the four-point probe test system is that it requires the destruction of specimens. Also, the distance between the four probes of the test system, which must make contact with the specimen, limits the lateral resolution of the test system. Similarly, X-ray fluorescence testing is also limited by large spot sizes. Additionally, X-ray fluorescence methods have difficulty distinguishing different film stacks thickness, generally results in poor measurements, and arc time consuming processes. It is important for the testing speeds to keep pace with the increasing fabrication speeds so that the goals of maximizing manufacturing throughput may be achieved. A specific problem with photo-induced surface acoustic wave methods is that they have difficulty resolving the thickness of copper layers; copper being a common conductor used in semiconductors.
In light of the foregoing, a probe system capable of achieving high resolution for thin film thickness measurements, and achieving such high resolution for high conductance materials, would be desirable.
The present invention allows for a non-contact, high frequency, and high-resolution schemes for thin metal film and surface conductance measurements. The present invention further allows for thickness measurements of high conductance materials. The conductance of a film structure specimen is determined from measuring the signal that is reflected from the probe and the frequency of the reflected signal.
One aspect of the present invention pertains to a tank probe system that includes a tank probe and a measurement instrument. The tank probe includes an inductor, a capacitor, and a resistor. The measurement instrument is connected to the tank probe by a transmission line. The measurement instrument includes a signal generator that sends a driving signal to the tank probe, and a signal analysis module configured to detect and measure a reflected signal from the tank probe and the frequency of the reflected signal, wherein the conductivity of the material specimen is determined from the reflected signal and the frequency of the reflected signal. In one embodiment of the invention, the tank probe is formed of integrated circuits within a semiconductor substrate. Another aspect of the present invention pertains to a method of using the tank probe system to measure the conductivity of a material specimen.
These and other features and advantages of the present invention will be presented in more detail in the following specification of the invention and the accompanying figures, which illustrate by way of example the principles of the invention.